Shaping colloidal rutile into thermally stable and porous mesoscopic titania balls

High crystallinity and controlled porosity are advantageous for many applications such as energy conversion and power generation. Despite many efforts in the last decades, the direct synthesis of organic-inorganic composite materials with crystalline transition metal oxides is still a major challenge. In general, molecules serve as inorganic precursors and heat treatment is required to convert as-synthesized amorphous composites to stable crystalline materials. Herein, an alternative approach to the direct synthesis of crystalline polymer-metal oxide composites by using a spherical polyelectrolyte brush as the template system is presented. Pre-synthesized electrostatically stabilized rutile nanocrystals that carry a positive surface charge are used as inorganic precursors. In this approach, the strong Coulomb interactions between anionic polyelectrolyte brush chains and cationic crystalline rutile colloids, whose surfaces are not capped and therefore reactive, are the key factors for the organic-inorganic crystalline composite formation. Stepwise calcination first under argon and followed with a second calcination in air lead to the complete removal of the polymer template without collapse and porous rutile balls are obtained. The results suggest that any colloids that carry a surface charge might serve as inorganic precursors when charged templates are used. It is expected that this hierarchical route for structuring oxides at the mesoscale is generally applicable.